In general, scattered light is used for measuring defects on smooth surfaces. For this purpose, the surface of, for example, a wafer is illuminated with a laser beam. The scatter characteristics of different particle sizes are shown in FIG. 1. A small particle 2 as well as a large particle 3 are illuminated with a laser beam 1. ‘Small particle’ means that the particle diameter is much smaller than the wavelength used. ‘Large particle’ means that the particle diameter is approximately of the same size as the wavelength or a little larger. As shown in FIG. 1, small particles scatter in isotropic manner into the space. Large particles, on the other hand, generate strong backscattering; see also scatter ellipse 4.
All methods of detecting surface defects are based on the detection of the scattered radiation, while blanking out the primary light reflected by a mirror. The intensity of the scattered light usually lies several levels below that of the reflected primary light.
Patent application U.S. Pat. No. 5,377,002 describes an apparatus for measuring scattered light, wherein the scattered light is focused through a converging lens onto a photo detector. In this process, the converging lens defines the acceptance angle before the scattered light. Directly reflected light is blanked out.
Patent application RU 2064670 proposes to collect the scattered light by means of an elliptical mirror disposed in rotational symmetry around the incident beam. Due to the very large acceptance angle, this apparatus is particularly sensitive to small particles.
Patent application WO 00/33055, as well, uses either ellipsoid or paraboloid, rotationally symmetrical mirrors, in order to collect the scattered light. However, the reflected primary light is not blanked out, but the scattered light is deflected via a deflection mirror. The incident and the reflected primary beams enter through an opening in the deflecting mirror. This apparatus, as well, is particularly sensitive to small particles.
Patent application EP 0624787 A1 proposes, in order to enhance the measuring sensitivity for large particles as well, to place two converging lenses in the path of the primary light beam within the ellipsoid rotationally symmetrical mirror, in order to be able to also detect the strong backscattering occurring with large particles. The primary light is blanked out again in front of the detector. This apparatus however has the disadvantage that scattered light is generated also at both converging lenses, which as a whole corrupts the measuring results. Furthermore, the focus is always on measuring the scattered light in its entirety, rather than on differentiating it based on particle size.
Since with all detection methods described the light spot can always only illuminate a fraction of the sample surface, the entire sample surface has to be scanned. This may be achieved with a rectangular grid, for example. With circular samples, such as wafers, a spiral-shaped scanning path as described in the patent application U.S. Pat. No. 4,314,763 is best suited. With this method, the sample is either rotated around its axis while simultaneously undergoing a translational movement in radial direction and while the light beam remains stationary, or the sample remains immobile and the spiral-form movement is executed by the light beam, which in very sensitive optical systems impairs the measuring accuracy.
Patent application WO 00/33055 further develops the spiral scan principle into the so-called record player principle. With this method, the sample surface is rotated around a first rotational axis. The light beam, meanwhile, travels on an arc around a second rotational axis. It swings over the sample surface just like the sensing head of a record player. This record player principle has prevailed on the market and is already used in the process-oriented quality surveillance for semiconductor components (e.g. device “Reflex 300” of the “Reflex” company, Moscow).
The apparatus already on the market has the disadvantage that the measuring head has to be kept very small, in order to allow for the swinging motion. This leaves very little space for the optical system so that it has to be very simple. For example, only very simple miniature diode lasers can be used as they work within the red spectral range. Since blue lasers require more space, the resolution cannot be further improved in these devices.